This invention relates generally to pulse radar operation, and more specifically, to methods and systems that provide an adaptive threshold for beam sharpening within a pulse Doppler radar.
There have been recent developments, for example, relating to the control of a detonation altitude in pulse radar equipped munitions. At least one of these developments utilizes beam sharpening with respect to transmissions from the munitions. This development makes use of the Doppler component on the radar signal to sharpen the beam which limits the forward view of the missile such that it looks along a velocity vector of the falling missile. Conceptually, sharpening the beam involves tuning a band-pass filter to the expected velocity of a missile, which is typically derived inertially within the missile. Objects to the side of the missile have lower Doppler frequencies and therefore a bandwidth of the filter limits the extent that the radar sees to the side of the missile.
Once the weapon reaches a specified reference altitude, which in at least one application involves using a range gate for processing the radar returns, the return Doppler signals are processed by integrating these return signals until a tracking threshold is reached. Once the returns are indicative of a valid tracking signal, then by knowing the reference altitude, the velocity of the weapon, and the preset detonation altitude, timing can be predicted as to when the detonation signal must trigger (e.g., provide) a fusing signal.
Accurate timing of the detonation signal is dependent on the integration time of the received Doppler signals and knowing when the valid tracking threshold has been achieved. Typically, a continuum of Doppler frequencies are received across a surface area of the field of view of the Doppler processor. The maximum Doppler frequency and amplitude occurs directly underneath the weapon system (i.e. maximum closing velocity). The other Doppler frequencies, which occur at lower frequencies, and their corresponding amplitudes contribute to an integrated solution in the Doppler filter, which is typically a band pass filter.
A problem arises due to the nature of band pass filters. The time delay through band pass filters varies as a function of the input frequencies. In particular, frequencies an octave or two from the pass band edges of the filter have much faster rise times than the desired center although their amplitudes are reduced. Since the radar will see a continuum of Doppler frequencies and because it has a very high sensitivity, the munitions may detonate on these side components resulting in errors in the detonation altitude.
Additionally, tuning the filter such that the upper pass-band edge corresponds to the Doppler frequency of the vertical component of the missile velocity is not an acceptable solution because of the additional 3 dB loss and the filter no longer acts as an integrator of Radar pulses. Coherent integration of the radar return pulses is needed to achieve sufficient sensitivity and reduced susceptibility to jamming.